1. Field of the Invention
The present invention relates to activated carbon and a method for producing it. More preferably, the invention relates to activated carbon obtained by activating a carbonaceous material followed by processing it with an acid, and to a method for producing it. The activated carbon of the invention has a strong affinity for water and is particularly suitable to water purification. In addition, as its electrostatic capacity is large, the activated carbon is also particularly suitable for electrodes for electric double-layer capacitors.
2. Description of the Related Art
Activated carbon is widely used in various fields such as the food industry, chemical industry and medicine industry, etc. The absorbability of activated carbon is useful in those fields wherein adsorbents having more absorbability are desired. In addition, and more recently, electric double-layer capacitors are becoming more important for backup power and power assist, and many developments concerning the capability of activated carbon for electrodes to electric double-layer capacitors are being made in the art. With the development of electronics, electric double-layer capacitors that contain polarizable electrodes of activated carbon are now in great demand, since their electrostatic capacity is large. Recently, such electric double-layer capacitors are used even in large-scale power assist devices, for example, in motors and the like, in addition to conventional systems such as back-up power sources for micro computer memories.
The principle of electric double-layer capacitors was known from the past, but their use in practical devices is relatively recent, and this use is increasing. The electrostatic capacity of electric double-layer capacitors is typically governed by the surface area of the polarizable electrodes therein to form electric double layers, by the electric double-layer capacity per unit surface area and the resistance of the electrodes. In the practical application of such electric double-layer capacitors it is important to increase the density of the electrodes therein for increasing their electrostatic capacity per unit volume and for reducing their volume. For electric double-layer capacitors, for example, heretofore used are (1) activated carbon prepared by activating a carbonaceous material of resin, coconut shell, pitch, coal or the like in an acidic condition, for example, In steam or acidic gas (see Technologies and Materials for EDLC, published by CMC in 1998); and (2) activated carbon prepared by activating the above starting materials with a chemical of high oxidizability, such as potassium hydroxide or the like (see WO91/12203, JP-A-199767/1998), etc.
Activating a carbonaceous material with an alkali compound and using it for polarizable electrodes for electric double-layer capacitors is known. For example, JP-A-139865/1989 discloses heating carbon fibers in an inert gas atmosphere at a temperature higher than 500xc2x0 C. in the presence of an excessive amount of an alkali metal hydroxide to prepare carbon fibers having a large surface area. JP-A-258996/1993 discloses an electrode for electric double-layer capacitors, which is produced by activating carbon fibers (these are prepared by melt-spinning a starting material of pitch followed by heating the resulting fibers) with an aqueous solution of an alkali metal hydroxide, then deashing, grinding and shaping them into electrodes. JP-A-161587/1995 discloses an electrode for electric double-layer capacitors, which is produced by activating a carbonaceous material with steam and then with an alkali metal hydroxide followed by grinding and shaping it into electrodes.
As mentioned above, it is desirable to increase the absorbability of adsorbents and to increase the electrostatic capacity of electrodes for capacitors. However, the activated carbon produced in the above-described method (1) could not ensure high electrostatic capacity, and it must be in large devices in order to ensure the necessary electrostatic capacity. On the other hand, in the above-described method (2), the starting carbonaceous material is activated with an activator of high oxidizability, such as potassium hydroxide or the like at a high temperature. The activated carbon obtained therein could have high capacity in its degree, but it is still unsatisfactory for the recent requirement for higher capacity. The materials disclosed and proposed In JP-A-139865/1989, JP-A-258996/1993 and JP-A-161587/1995 are also not satisfactory.
Accordingly, one object of the present invention is to provide activated carbon having the advantage of high absorbability for impurities dissolved in water and the advantage of large electrostatic capacity, and to provide a method for producing it.
This and other objects have now been attained with the present invention, the first embodiment of which provides an activated carbon produced by a process, which includes:
activating a carbonaceous material, to obtain an activated carbonaceous material; and
contacting the activated carbonaceous material with an acid.
Another embodiment of the present invention provides an activated carbon, having substantially the same IR spectrum as set out in FIG. 1.
Another embodiment of the present invention provides an activated carbon, having substantially the same IR spectrum as set out in FIG. 2.
Another embodiment of the present invention provides an activated carbon, having substantially the same IR spectrum as set out in FIG. 3.
Another embodiment of the present invention provides an activated carbon, which includes at least one IR absorption peak in the range of 1600 to 1800 cmxe2x88x921.
Another embodiment of the present invention provides an electrode for an electric double-layer capacitor, which includes the above-described activated carbon.
Another embodiment of the present invention provides a filter, which includes the above-described activated carbon.
Another embodiment of the present invention provides a shaped article, which includes the above-described activated carbon.
Another embodiment of the present invention provides a method for producing activated carbon, which includes activating a carbonaceous material, to obtain an activated carbonaceous material; and contacting the activated carbonaceous material with an acid, to obtain the activated carbon.